1. Field of the Invention
This invention relates to a process for conducting an equilibrium reaction in which one or more products is/are separated off by vapor permeation. More particularly, the invention relates to a process for conducting an esterification reaction in which water vapor is removed by vapor permeation.
2. Statement of Related Art
Membrane processes for the fractionation of mixtures containing organic components are described in detail in the literature, cf. for example Rautenbach et al. in Chem. Ing. Techn. 61 (1989), pages 535-544. The use of pervaporation and vapor permeation is also described in the prior art literature, cf. DE 3 610 011 C2, EP 0 294 827 A2 and EP 0 273 267 A2.
In these processes, the mixture to be separated (feed) is generally guided along a membrane which is permeable to one component (semi-permeable). A vacuum applied to the feed-remote side (permeate side) of the membrane produces a potential gradient which results in removal of the predominantly permeating component from the feed. The stripped feed is called the retentate or concentrate. The substantially pure component separated by the membrane is called the permeate. The separation behavior of the membrane is very much dependent on temperature, although an upper limit is imposed by thermal stability. For example, the GFT standard membrane is said to have a maximum thermal stability of 130.degree. C. (G. F. Tusel et al., ACS Symposium Series 281 (1985), Reverse Osmosis and Ultrafiltration, S. Sourirajan (Ed.) 467-478). However, there is still no evidence of long-term stability above 100.degree. C. On the basis of operating experience, the optimal separation behavior of the GFT standard membrane is between 80.degree. C. and 100.degree. C.
The processes described in DE 3 610 011 C2 and EP 0 294 827 A2 are distinguished by the fact that the feed is introduced in liquid form just below its boiling point under the prevailing system pressure. The system pressure has to be selected so that the temperature of the feed at the membrane corresponds to the optimal temperature and is never above the maximum temperature because the membrane would otherwise be irreversibly damaged.
In addition, the contact of the feed with hydrophilic membranes (GFT standard membranes) results in swelling of the active layer which, in turn, causes an increase in permeability on the one hand, but a loss of selectivity on the other hand. With high water contents and high temperatures, this swelling can lead to separation of the active layer from its backing cloth and, hence, to destruction of the membrane. In addition, there must be no danger of chemical damage to the membrane by any one of the feed components.
Accordingly, the use of pervaporation in reactions is seriously limited.
The process of vapor permeation is described in EP 0 273 267 A2. Vapor permeation is distinguished from pervaporation by the fact that the feed is evaporated and the vapor is guided along the membrane. In this case, too, the system pressure has to be selected so that the vapor temperature and hence the temperature of the feed correspond to the optimal temperature of the membrane. In vapor permeation, too, the boiling temperature of the feed must not exceed the maximum temperature of the membrane.
Vapor permeation has the advantage over pervaporation that, on the one hand, the membrane does not swell and, on the other hand, the danger of chemical damage to the membrane is largely ruled out.
U.S. Pat. No. 2,956,070 and EP 0 210 055 A1 are concerned with the removal of water during esterification reactions by pervaporation.
For the reasons explained above, pervaporation is carried out at moderate reaction temperatures (50.degree.-90.degree. C.) in the tests mentioned in EP 0 210 055 A1. In this case, reference is made to the low acid resistance of the membranes (max. 15 mol-% of the reaction mixture) which causes a large excess of alcohol.
A process for using vapor permeation for the removal of water in the esterification of oleic acid with ethanol is described in Kita et al., Chem. Letters (1987), No. 10, pp. 2053-2056. In this case, the tests were carried out at moderate temperatures (85.degree. C.). In the test arrangement used, the reaction temperature again must not exceed the maximum temperature of the membrane for the reasons explained above.